What is skeletal light scattering?

 

When sunlight enters the tissue of a coral and is not absorbed or scattered by the tissue, associated pigments, or symbiont algae, it will be scattered by the skeleton in a predictable manner dependent on the architecture of the micrograins of calcium carbonate that the coral deposited during construction of the skeleton. The diversity of micro architecture that we have observed in 96 taxa of corals results in a continuum of light scattering ranging from 'high scattering corals' that scatter most photons directly back at the observer (Fig. b) to 'low scattering corals' which transport light through the skeleton (Fig. d). While both ends of the continuum can redirect unabsorbed light back to the symbiont algae and enhance photosynthesis, low scattering corals do so at a greater rate. This increased rate of light delivery to the symbiont algae could be beneficial to photosynthetic efficiency under normal conditions, however as the bleaching response is initiated and absorbing units are lost, excess light experienced by the symbiont algae quickly becomes overwhelming and compounds thermal stress with light stress thereby precipitating the bleaching response. We have demonstrated a link between 'low scattering' skeletal architecture and bleaching response through assessment of the historical bleaching record, modelling bleaching corals with 'tissue phantoms', and experiments with live corals.  We will have more on these different techniques shortly!